High quality semiconductor wafers are required to produce reliable and high precision semiconductor devices. These high quality semiconductor wafers require an extremely flat surface on at least one side to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. CMP is often used to remove material from the wafer surface to provide a relatively flat surface before building devices on the wafer surface. In addition, CMP is also used for interlevel dielectric planarization and metal polishing during the formation of the devices on the wafer surface. CMP is well known in the art and generally includes placing one side of the wafer in contact against a flat polishing surface, and moving the wafer and the polishing surface relative to one another. In addition, a slurry which includes abrasive particles and/or chemicals that react with the material on the wafer surface to dissolve the surface material may also be introduced between the wafer surface and the polishing pad to assist in removing a portion of the surface material. During the polishing or planarization process, the wafer is typically pressed against a rotating polishing pad. In addition, the wafer may also rotate and oscillate back and forth over the surface of the polishing pad to improve polishing effectiveness.
As previously stated, chemical mechanical polishing is performed on the wafer surface several times during the fabrication of a semiconductor device. Semiconductor manufacturers often measure the wafers before and during the formation of semiconductor devices. Manufacturers measure the wafers to ensure that the within wafer nonuniformity, the removal rate, and the removal rate stability are within process specifications. Off-line measurements tend to dominate the current form of measurement. As a result, semiconductor manufacturers can lose several hours per shift processing and measuring the monitor and product test wafers off-line. This can result in reducing, by almost one-half, the CMP equipment capacity.
Accordingly, a great deal of interest has been shown in in-line and/or in-situ measurements to assess CMP processes. In-line and/or in-situ measurements can assess the quality of the monitor and product wafers either immediately after (in-line) or during (in-situ) the polishing of the wafers and can thereby reduce or nearly eliminate the time typically required to test the wafers off-line. As a result, semiconductor manufacturers have been searching for a viable method of either monitoring the planarization of the surface or monitoring the removal rate of the dielectric form the surface of the wafer. In addition, manufacturers have also been searching for improved metal endpoint detection systems.
Multiple in-situ endpoint detection approaches have been proposed and tested for use in CMP including optical, electrical and acoustic sensing. Optical techniques used for detecting endpoint in CMP are primarily interferometry, reflectance, and spectral reflectivity . In U.S. Pat. No. 5,081,796, interferometry is used to measure a wafer which overhangs the edge of a platen and in relation to an unpatterned die. An alternative approach for endpoint detection using interferometry involves inserting small interferometers into a CMP carrier. In still another approach, an optical window is embedded in the rotating polishing pad and the platen to enable in-situ viewing of the surface being polished. Specular reflection from a test surface strikes a detector. The intensity of the reflected light changes markedly as the film thickness approaches zero. This method is described in U.S. Pat. No. 5,433,651. Further, in U.S. Pat. No. 5,196,353, an infrared camera positioned slightly below or with the top of the polishing pad senses the temperature at the surface of the wafer. Endpoint is detected by the temperature change which occurs when the friction changes in passing from one material to another.
Methods for detecting endpoint using electrical measurements include those that sense friction and those that do not. The electrical measurement methods that do not sense friction typically require electrical connections to the wafer during CMP or modifications to the platen and/or carrier assembly which affect the basic performance of the tool. The electrical measurement methods that do sense friction work particularly well in metal CMP since the CMP eventually leads to the exposure of the underlying interlevel dielectric layer which has a considerably different coefficient of friction than the metal. In contrast, planarization of the topography on an interlevel detection layer does not involve a transition to an underlying layer with a different coefficient of friction and therefore is not as amenable to this approach.
There have been several patents related to measuring changes in friction to detect endpoint. For example, U.S. Pat. Nos. 5,036,015 and 5,069,002 detect endpoint by monitoring changes in the motor current to infer the state of friction between the wafer and the pad. In this method, the motor current changes when one material having a given coefficient of friction is polishes through to an underlying material with a different coefficient of friction. Further, U.S. Pat. No. 5,308,438 monitors the motor current of the platen to track the power required to rotate the platen which is based on the coefficient of friction of the surface being polished. For example, when a surface having a low coefficient of friction such as metal is polished, the motor current is also low. The motor current then rises as the thickness of the metal film goes to zero and the polishing pad begins to polish the oxide.
The acoustic methods for detecting endpoint are based on the idea that the grinding action taking place during polishing generates an acoustic signal. These methods include monitoring the change in amplitude and frequency of spectral peaks as well as analyzing acoustic wave velocity.
Finally, a method for detecting endpoint by monitoring the electrochemical potential of the system is described in U.S. Pat. No. 5,637,185. In this method, the difference between a measurement electrode and a reference electrode are measured. The measurement electrode could be either the polishing surface or a probe inserted into the slurry near the wafer being polished while the reference electrode could be a saturated calomel electrode or a standard hydrogen electrode.
Although several approaches do exist for detecting in-situ endpoint in CMP, not all have become commercially viable and even those that are commercially viable have not proven to be 100% reliable. Accordingly, there is a need for additional systems and methods for determining in-situ endpoint in CMP. There is also a need for a system and method for detecting in-situ endpoint in CMP that reduces cost of ownership while increasing the reliability of endpoint detection.